1. Technical Field of the Invention
This invention broadly relates to well cementing. The invention further relates to well plugging. The invention more particularly relates to a method of forming a cement plug in a well bore.
2. Description of the Prior Art and Problems Solved
The term, xe2x80x9cprimary cementing,xe2x80x9d is employed by persons skilled in the art of well cementing to refer to the formation of a sheath of cement in the annular space between the wall of a bore hole drilled in the earth and the exterior wall of a casing positioned in the bore hole. The sheath is ordinarily formed as a part of the initial construction of a well, such as a well which produces hydrocarbons, for example, liquid petroleum and natural gas, from a subterranean earth formation. The purpose of the sheath is to stabilize the bore hole, support the casing in the bore hole and segregate subterranean formations which contain hydrocarbons from subterranean formations which contain water, particularly potable water. The sheath of cement can extend from the bottom of the casing to the surface of the earth. A method of forming the sheath is well known.
The term xe2x80x9ccasingxe2x80x9d is employed in the previous paragraph to broadly refer to at least one and usually two or more tubular conduits of decreasing diameter which, together in a telescoping mode, extend from the surface of the earth to the bottom of the bore hole. In one typical example, a so-called surface casing having a large diameter continuously extends from the surface of the earth to a point below the deepest formation which contains potable water. A second casing, sometimes called a production casing whose outside diameter is less than the inside diameter of the surface casing, continuously extends from the surface of the,earth to a target formation, such as one which contains hydrocarbons. A sheath of cement is placed in the entire annular space between the surface casing and the bore hole and a second sheath of cement is placed in the annular space from the bottom of the production casing to a point above the target formation.
When the producing life of a well is complete, such as when recovery of hydrocarbons from the well is no longer economically sound, the well is abandoned. Abandoned wells pose a variety of hazards, one of which is the potential of undesirable fluids, such as hydrocarbons and/or salt water, which originate from subterranean formations penetrated by the bore hole, to migrate to and contaminate potable water in other subterranean formations which are also penetrated by the bore hole. To prevent such contamination, regulatory agencies in the several states require that abandoned wells be plugged, such as by placing a mass of hardened cementitious material in the well bore at least at points adjacent hydrocarbon producing formations and also at points adjacent potable water formations. Such plugs completely occupy the well bore volume adjacent the formations and function as a barrier to migrating fluids.
The current method of plugging a well broadly comprises forming a slurry of cement in water at the well head, introducing a continuous connected string of delivery pipe (sometimes called tubing), into the well bore until the bottom of the tubing attains a desired point of delivery of the slurry, pumping the slurry down the tubing to the bottom thereof and back up the exterior thereof, continuing pumping until a desired quantity of slurry has been deposited in the well bore to form a plug therein, and then withdrawing the tubing from the well bore. In the current method when the tubing is withdrawn from the well bore, the top of the slurry on the exterior of the tubing is preferably at the same level as the slurry in the interior of the tubing. This is referred to as a balanced plug. Before a second balanced plug can be placed, the cement in the preceding plug must first be permitted to set to a minimum hardened condition. Accordingly, if multiple plugs are required, then they cannot all be placed in a continuous operation due to the need to wait on cement to set.
Note use of the terms xe2x80x9cbore holexe2x80x9d and xe2x80x9cwell bore.xe2x80x9d For purposes of disclosure, the term xe2x80x9cbore holexe2x80x9d is employed to describe the linear hole actually drilled in the earth. The wall of the bore hole is the earthen rock exposed by the drill. The term xe2x80x9cwell borexe2x80x9d is employed to describe the containment vessel for the conduit or the intended conduit through which fluids pass between the surface of the earth and subsurface formations penetrated by the bore hole. It is common to install a continuous string of casing in the interior of the bore hole. The volume of the interior of the casing is the well bore. The volume between the wall of the bore hole and the exterior surface of the casing is referred to as the annular space. Thus, primary cementing involves placing cement in the annular space and well plugging involves placing cement in the well bore. In the absence of a casing it is clear that there is no annular space and there is no distinction between bore hole and well bore.
Persons skilled in the art know that considerable surface equipment is required to perform the current method of well plugging. Such equipment comprises a derrick to suspend tubing in the hole, transports for delivering to and storing dry cement and water at the well head, equipment at the well head for blending and mixing the cement and water to form the slurry and a high volume/high pressure pump at the well head to pressure the slurry down the suspended tubing and back up the exterior thereof to a predetermined destination.
The current method is employed to produce cement plugs in wells regardless of depth, and is particularly useful to form plugs in wells whose internal pressures are sufficiently high to cause fluids to naturally flow to the surface of the earth. Such wells require the use of methods and equipment which function to control such pressures and to prevent the flow of fluids from the well while the cementing operation is proceeding.
A need thus exists for a method of forming cement plugs in wells whose internal pressures are not sufficiently high to cause formation fluids to flow to the surface of the earth.
By this invention there is provided a method of well cementing which comprises forming a plug of cement in the well bore. According to the method of the invention, a suitable bore hole is first selected. Upon selection of a suitable bore hole, a liquid comprising a cementitious slurry is introduced into the well bore at the surface of the earth. The liquid is permitted to descend in the well bore by gravity, form a column of slurry to a desired point in the well bore and then permitted to harden therein to form a plug. The bulk density of the introduced liquid is selected so that it, when multiplied by the distance from the surface of the earth to a designated location in the well bore, produces a pressure which is in excess of the natural pressure at the face of any subsurface formation actually contacted by the liquid. The method of this invention thus depends upon hydrostatic pressure generated by introduced liquid and not on mechanical pressure generated by a surface pump.
A suitable bore hole is one which penetrates at least one subsurface formation which produces a well fluid other than fresh water, wherein the natural pressure of the formation is not great enough to cause the well fluid to flow from the formation through the bore hole to the surface of the earth. The formation must possess sufficient permeability and porosity to permit the well fluid to be injected into it within an acceptable period of time by pressure induced at the formation face by hydrostatic pressure in the well bore and the formation must also possess sufficient structural strength to avoid being fractured by such induced pressure.
The method of this invention features positioning all plugs required within the entire well bore in one continuous operation without stopping to wait for a preceding plug to set. Thus, the reference in the preceding paragraph to an xe2x80x9cacceptable period of timexe2x80x9d of injection of well fluid into a subsurface formation is the amount of time that a slurry must remain in a flowable liquid state before it begins to set. It is believed that such an xe2x80x9cacceptable period of timexe2x80x9d is in the range of from about 8 to about 10 hours. A set time in the range of 8 to 10 hours can be selected by the addition to the slurry of known set time additives.
As mentioned, the formation into which well fluid is injected must also possess sufficient structural strength to avoid being fractured by the total hydrostatic pressure produced at the formation. In this regard, if the total hydrostatic pressure at a formation divided by the distance to the formation from the surface, i.e. the pressure gradient, is a value in the range of from about 0.4 to about 0.5 lb/sq.in. per foot of depth, then it is believed that a fracture will not be produced in the formation. For example, a liquid having a bulk density equal to the density of water (62.43 lb/cu.ft.) in a well having a depth of about 6500 feet produces a pressure gradient of about 0.4335 lb/sq.in. per foot of depth.
In addition to the low probability that a fracture will be induced by the hydrostatic pressure created by liquid in a well that is less than or equal to about 6500 feet deep, the well bore temperature in such wells is considered by persons skilled in the art to be low. Minimal performance requirements are demanded of cement at low temperature applications, so a wide range of cement compositions will operate.
Accordingly, by the method of this invention, a suitable well is first selected. Such a well is one which is no longer productive of useful well fluids, such as oil and gas, and has a subsurface formation containing such well fluids which is penetrated by a bore hole. The term well fluid can also include water produced from the formation which is sometimes referred to as produced formation water. The formation is not blocked by any device in the bore hole and, thus, has unobstructed access to the surface of the earth via the well bore. The natural pressure in the formation is not great enough to cause well fluids to flow to the surface of the earth. Such a condition can be evidenced by a static column of well fluid in the well bore which does not reach the surface of the earth.
Having thus located such a candidate well, the next step in the selection method is to determine whether the formation has sufficient porosity and permeability to accept low viscosity fluids in an acceptable period of time without undergoing a fracture. Accordingly, a simple injectivity test is performed by filling the well bore to the surface of the earth with a measured quantity of a fluid having a known density and a known viscosity; permitting fluid to flow into the mentioned formation; measuring the time required for fluid in the well bore to attain a static condition; and measuring the level of the attained static column of fluid. The measured quantity of fluid introduced into the well bore is that quantity which is equal to the volume of the well bore between the formation and the surface of the earth less the quantity of the static column of well fluid initially present therein. The quantity of fluid which actually enters the formation is determined by appropriate mathematical combination of fluid in the well bore before the test, the quantity of fluid in the well bore after the test and the measured quantity of fluid introduced into the well bore during the test. The density of the fluid added during the test is at least equal to, and is preferably greater than, the density of the fluid initially at rest in the well bore. The natural pressure within the formation can be calculated by those skilled the art by use of the density of the well fluid and the height of the static column of fluid above the formation. The porosity and permeability of the formation is then determined by application of, for example, the D""arcy Equation which is known by those skilled in the art of reservoir evaluation.
Having thus selected a candidate well, the method of this invention is further comprised of forcing at least a portion of the well fluid initially standing in the well bore from the well bore into the subsurface formation or formations of its source while, simultaneously, entirely replacing such portion, in one aspect, with a single quantity of a first liquid comprising a cementitious material, or, in a second aspect, with a combination of the first liquid, a second liquid and dense spacing discs or plugs followed by a single quantity of the first liquid.
The combination of first liquid, second liquid and dense spacing discs or plugs is defined herein as xe2x80x9ca cementing unitxe2x80x9d which consists of two spacings discs, a single quantity of first liquid and a single quantity of second liquid, wherein the spacing discs are placed between successive quantities of second liquid and, first liquid or between well fluid and first liquid as the case may be.
The single quantity of first liquid is defined herein as xe2x80x9ca final unitxe2x80x9d which consists of a single quantity of cementitious material and one spacing disc.
In the mentioned second aspect, the method of this invention is comprised of a series of steps which operate to force the well fluid into the subsurface formation or formations of its source in stages by employing at least one cementing unit and a final unit, wherein at least one cementing unit is employed per subsurface formation containing an undesirable well fluid. The final unit is used to block formations containing potable water. The entire well bore from the bottom thereof to the surface is filled with the cementing units and the final unit.
Guided by the known relationship that pressure is the product of height and density, to create hydrostatic pressure sufficient to force the well fluid into the formation the bulk density of the cementing units and final unit can be equal to or greater than the bulk density of the well fluid. Furthermore, the hydrostatic pressure created by the weight of the combination of cementing units and final unit at the formation must be greater than the natural reservoir (pore) pressure of the formation. Methods of preparing the first liquid, which is comprised of cementitious material, and the second liquid, which is a spacer fluid, each having a desired density, are well known in the art of well cementing.
Descriptions of the cementitious materials, spacer fluids and spacing discs as well as a more detailed account of the steps employed in the method of this invention are provided below in connection with the drawings and appended example.